How do afterburners work

You can investigate nozzle operation with our interactive nozzle simulator. The nozzle of a turbojet is usually designed to take the exhaust pressure back to free stream pressure. The thrust equation for an afterburning turbojet is then given by the general thrust equation with the pressure-area term set to zero. If the free stream conditions are denoted by a "0" subscript and the exit conditions by an "e" subscript, the thrust F is equal to the mass flow rate m dot times the velocity V at the exit minus the free stream mass flow rate times the velocity.

This equation contains two terms. For clarity, the engine thrust is then called the net thrust. Our thrust equation indicates that net thrust equals gross thrust minus ram drag. Afterburners are only used on fighter planes and the supersonic airliner, Concorde. The Concorde turns the afterburners off once it gets into cruise. The back portion of the turbine acts like a windmill , extracting energy from the exhaust gases and using the energy to spin the compressor portion.

See How Jet Engines Work for details. A modern turbine engine is extremely efficient, and there is still a lot of oxygen available in the exhaust stream. The idea behind an afterburner is to inject fuel directly into the exhaust stream and burn it using this remaining oxygen.

The big advantage of an afterburner is that you can significantly increase the thrust of the engine without adding much weight or complexity to the engine.

An afterburner is nothing but a set of fuel injectors, a tube and flame holder that the fuel burns in, and an adjustable nozzle. A jet engine with an afterburner needs an adjustable nozzle so that it can work both with the afterburners on and off. The disadvantage of an afterburner is that it uses a lot of fuel for the power it generates.

Therefore most planes use afterburners sparingly. For example, a military jet would use its afterburners when taking off from the short runway on an aircraft carrier, or during a high-speed maneuver in a dogfight. The following pictures, taken at the Virginia Air and Space Museum, show you some of the details of an afterburner-equipped engine. The afterburner is designed so that the flame flows along its axis, away from its walls. Careful placement of the fuel tubes and the ignition source at the front end of the jetpipe the four- to seven-foot-long tube at the back of the engine , where hot but not burning exhaust gas is flowing out of the engine, creates a stable zone in the airflow where air and fuel can mix.

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